This invention relates to a method apparatus for forming a film and more particularly to a method and an apparatus suitable for forming a film of semiconductor, dielelctric, metal, insulator, or organic substance.
It is well known that sputtering is conventionally used for forming a film of metal or dielectric material. As discussed in pages 171 to 195 of "Hakumaku (Thin Film) Handbook" edited by Japan Society for the Promotion of Science (published by Ohmsha, Ltd., December 1983), there are wide range of methods which include DC diode sputtering, bias sputtering, radio-frequency (RF) sputtering and magnetron sputtering, the last one being intended for high-speed film formation. In addition, an ion beam sputter deposition method is also known, which is described in pages 190 and 191 of the above-mentioned literature.
All of these methods utilize sputter ejection of atoms from a substance in the solid state at room temperature which occurs when it is struck by an ion beam or ions in the plasma. In this case, the sputtering mechanism is explained by collision cascade model. Therefore, the film forming speed is limited by the sputtering rate of a substance in the solid state.
On the hand, sputtering is achieved by ion irradiation of a substance in the solidified state made by cooling a gas of carbon monoxide, methane, etc. and combing their molecules by van der Waals forces. As an example, this sputtering method is discussed in "Desorption Induced by Electronic Transition DIET II" pp. 170-176 (compiled by W. Brening, D. Menzel, published by Springer, 1985).
In this case, it is known that the number of atoms and molecules which are decomposed per incident ion by sputtering of a solidified substance of a gas of carbon monoxide or methane, that is to say, the sputtering yield is about 100 to 1000.
Japanese Patent Application JP-A-63-177414 discloses a method in which laser light is irradiated on a gas or a solidified substance to quickly impart energy to this substance in order to convert it into a plasma and a film is formed by using particles produced in the plasma.
Among the prior-art techniques, the sputtering method employing ions and a plasma uses a substance in the solid state at room temperature as a target material for sputtering and forms a film by sputtered particles which are decomposed and sputtered by a sputtering mechanism described in pages 171 to 179 of the above-mentioned "Thin Film Handbook". In this case, the sputtering yield of the target per ion incident on the target is 1 to 10. Accordingly, the problem with this method is the slow film forming speed of 0.1 to 1 nm/sec. In principle, the purity of the deposited film never becomes higher than that of the target for sputtering. When a high-purity target for sputtering is not available, you can obtain only a film of poor quality which contains impurities, which has been another problem.
In a film forming method using a laser-induced plasma, the mechanism whereby plasma particles are produced is such that the target absorbs the energy of photons. Therefore, the kinetic energy of a plasma particle thus produced is smaller than that of a particle sputtered from the target by irradiation by ions having large momentum. For this reason, the deposited film is inferior in adhesion strength to those which are formed by the conventional sputtering methods in which irradiation by ions having large energy is used.
Meanwhile, in depositing an organic film, the wet process has been used chiefly. The feature of this wet process is that a relatively uniform film is formed easily as in the spin coating method and the cast method. Therefore, the wet process is superior in productivity and is applied extensively in film forming processes such as applying a photoresist for photolithography, an insulating film between layers of a semiconductor element, or a liquid crystal arrangement control film of a liquid crystal display element.
In contrast to the foregoing, the vacuum evaporation technique is a dry process in film deposition. There was a report on the formation of an organic single crystal film in "Journal of Applied Physics", volume 36, number 4, April 1965, pp. 1453-1460. According to this method, it is possible to form a film with superior crystallinity and orientation.
The plasma polymerization method is another film forming technique by a dry process. For example, a report on a metallic phthalocyanine plasma-polymerized film was carried by "Journal of the Chemical Society of Japan, No. 11 (1987), pp. 2019-2024. By this method, it is possible to form not only a functional film with an ability of photoelectric conversion, etc., but also a film generally superior in mechanical strength and heat resistance.
In the meantime, the organic film forming method by a wet process has mass-production aptitude and is currently the best film forming method. However, considering technical requirements for an organic film forming method to be used for the development of the next-generation devices, there are some requirements which are hard to meet by the conventional wet processes.
For example, the liquid crystal display needs to meet requirements for further microminiaturization, color display and an enlarged screen. Efforts are being actively exerted to develop a liquid crystal display by the active matrix method, which combines display elements and a driving circuit. In one display, there are tens of thousands to hundreds of thousands of transistors formed on a glass plate several tens of centimeters square. Therefore, it is required to form a uniform photoresist film in the transistor manufacturing process. In the spin coating method, to secure a uniform film thickness, the substrate is rotated at about 3,000 revolutions per minute and a photoresist is applied thereto. As the substrate increases in size, the edges of the substrate turns at a high speed so high as to approach the speed of sound. By technology at the moment, it is impossible to rotate and hold a substrate stably and apply the resist uniformly.
With regard to an insulating film between the layers of a semiconductor element, a problem with the wet process has been pointed out. A polyimide film for use as an insulating film is conventionally made as follows. A diamino compound and tetracarboxylic acid (or its derivative) are polymerized in an organic polar solvent to make a polyamide acid varnish. The polyamide acid varnish is spread across the substrate, then the substrate is heated and dried to obtain a polyimide layer formed on the surface. (Refer to JP-B-36-10999, for example.) With growing needs for higher integration of semiconductor devices as the backdrop, it is required to provide sufficient insulation performance with a film about 1 .mu.m thick. The method mentioned above is unable to eliminate pinholes completely. There is also a problem that mixing-in of impurities is liable to occur. Therefore, a film that can withstand practical application has not been realized by this method.
An organic lubrication layer is formed, unquestionably, by a wet process, to prevent the wear of the surface of the magnetic disk. In order to respond to the needs for an improvement in recording density, the magnetic recording layer, which used to be formed by a wet process, has come to be formed by a dry process. Accordingly, it has become necessary to adopt a dry process for the formation of a lubrication layer to maintain the quality of the film formed and enhance the efficiency of film formation.
In forming an organic film by a dry process, which is to replace the wet process, the vapor evaporation method and the plasma polymerization method have advantages as described above. However, the former method has problems that it is difficult to form a film with a large area, that the film forming speed is slow, and that the number of substances that can be used for vacuum evaporation is limited (for example, the substances which are subject to a polymerization reaction when they are heated for evaporation cannot be used). In the latter method, disorder is liable to occur in the molecular composition and arrangement, and it is difficult to control the molecular chain orientation.